1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a device and a method for transmitting and receiving data by a transmit diversity scheme using multiple antennas.
2. Description of the Related Art
Next-generation mobile communication systems have been developed to form a packet service communication system which transmits burst packet data to multiple mobile stations. Packet service communication systems have been designed to be suitable for large data transmission and to provide high-speed packet services. The 3rd Generation Partnership Project (3GPP), which is an asynchronous mode standards group, suggests a high speed downlink packet access (“HSDPA”) for high-speed packet services. Also, the 3rd Generation Partnership Project 2 (3GPP2) synchronous standards group suggests an 1× Evolution Data Only/Voice (1×EV-DO/V) technique to provide high-speed packet services. The HSDPA and 1×EV-DO/V techniques both provide high-speed packet services in order to match Internet services such as the web. For high-speed packet services, both techniques optimize the average throughput and the peak throughput to enable smooth transmission of packet data, as well as circuit data such as voice.
Hereinafter, HSDPA will be described in more detail.
Generally, HSDPA scheme refers collectively to a high speed downlink shared channel (“HS-DSCH”), which is a downlink data channel for supporting downlink high-speed packet data transmission in a Wideband-Code Division Multiple Access (W-CDMA) communication system, related control channels, and an apparatus, system or method for the channels. Although HSDPA in the 3GPP asynchronous standards is explained herein, the present invention is applicable to any communication systems which implements transmit diversity using two or more transmit antennas.
In communication systems using HSDPA, three schemes, Adaptive Modulation and Coding (AMC), Hybrid Automatic Retransmission Request (HARQ) and Fast Cell Selection (FCS), have recently been introduced to support high-speed packet data transmission.
AMC is a data transmission scheme which improves the utilization of whole cells by selecting modulation and coding methods for different data channels according to the channel condition between cells, i.e., between the base station (Node B) and the user equipment (UE). The AMC scheme includes a plurality of modulation methods and coding methods and combines the modulation and coding methods to modulate and code data channel signals. Each combination of modulation methods and coding methods is called an Modulation and Coding Scheme (MCS). MCSs can have level 1 to level n according to the number of the MCSs. In other words, AMC adaptively determines the level of the MCSs according to the channel condition between the user equipment and the base station in an wireless network, thereby improving the overall system efficiency of the base station.
Secondarily, HARQ and more particularly n-channel Stop and Wait Hybrid Automatic Retransmission Request (N-channel SAW HARQ) will be explained in detail.
Common Automatic Retransmission Request (ARQ) exchanges an acknowledgement (ACK) signal and retransmission packet data between the user equipment and the radio network controller (RNC) of the base station. However, HARQ applies two new approaches to improve the transmission efficiency in ARQ. One is for performing a request for and a response to retransmission between the user equipment and the base station. The other is for temporarily storing data with errors, combining them with their retransmission data and transmitting the combined data. Also, HSDPA exchanges an ACK signal and retransmission packet data between the user equipment and the Media Access Control (MAC) HS-DSCH of the base station. HSDPA introduces the N-channel SAW HARQ scheme which forms logical channels in the number of N to transmit multiple packet data even when no ACK signal is received. The SAW ARQ scheme transmits next packet data only upon receiving an ACK signal for the previous packet data. Accordingly, there may be an occasion to wait for an ACK signal even at the moment when packet data can be transmitted. The N-channel SAW HARQ scheme can improve the utilization of channels by continuously transmitting multiple packet data without having received an ACK signal for the previous packet data. In other words, the N-channel SAW HARQ scheme sets logical channels in the number of N between the user equipment and the base station. If each of the N logical channels can be identified by a particular time or channel number, the user equipment can determine the logical channel through which packet data received at a particular point of time was transmitted. Also, it is possible to rearrange packet data in the order in which the packet data should be received, or to soft-combine particular packet data.
FCS is a scheme for rapidly selecting a cell in a good channel condition, among a plurality of cells, when the user equipment (UE) using the HSDPA service is in a cell overlapping region, i.e., in a soft handover region. Specifically, when the UE using HSDPA enters a cell overlapping region of a first base station and a second base station, the UE sets up a radio link with a plurality of cells, i.e., a plurality of base stations. A group of cells where a radio link with the UE is established are called an “active set.” In order to reduce the overall interference, packet data for HSDPA is received only from a cell in the best channel condition, among cells in active set. The cell in the best channel condition in the active set is called a “best cell.” The UE periodically checks the channel conditions of the cells in the active set to determine whether a cell having a better condition than the current best cell has been generated. If a cell having a better condition than the current best cell is generated, the UE transmits a best cell indicator to every cell in the active set in order to set that cell as the new best cell. The best cell indicator includes an identifier of the cell selected to be the best cell. Cells in the active set receive the best cell indicator and detect the cell identifier included in the best cell indicator. Each cell in the active set determines whether the cell identifier included in the best cell indicator corresponds to itself. The cell selected as the best cell transmits packet data to the UE using HS-DSCH.
As explained above, communication systems using HSDPA suggest a variety of new schemes for improving data transmission rates. Although only the HSDPA scheme has been explained above, systems such as 1×EV-DO/V system are also provided to improve data transmission rates. The 1×EV-DO/V system is focused to improve data transmission rates. In addition to AMC, HARQ and FCS schemes, a multiple antenna scheme has been proposed to overcome the limitation of the assigned bandwidths and improve the data transmission rates. The multiple antenna scheme utilizes the space domain to overcome the bandwidth limitation in the frequency domain. Generally, a nulling algorithm is used in the multiple antenna scheme.
Before explaining the multiple antenna scheme in further detail, a multiuser diversity scheduling scheme will be explained. Packet service communication systems, for example, HSDPA communication systems, determine the conditions of multiple user channels based on feedback information and send packet data only to the user channel having the best channel quality, thereby increasing the signal-to-noise ratio (SNR) gain. This is the multiuser diversity scheduling scheme. The diversity order, which represents the degree of the multiuser diversity gain, corresponds to the number of users who require a packet service simultaneously.
Hereinafter, the multiple antenna scheme will be explained in further detail.
Mobile communication systems allow multiple items of user equipment to communicate with each another through a single base station. When the base station transmits data to the multiple user equipment at a high speed, channels become faded due to the wireless channel characteristics. As a solution to overcome the fading problem, a transmit antenna diversity scheme which belongs to the multiple antenna scheme has been proposed. The transmit antenna diversity scheme transmits signals using two or more multiple antennas to minimize the loss of transmitted data caused by fading and to enhance the data transmission rates. The transmit antenna diversity scheme will be explained below in more detail.
Unlike the wire channel environment, the wireless channel environment in a mobile communication system receives signals that have become distorted from originally transmitted signals due to various factors, such as multipath interference, shadowing, radio wave attenuation, time-varying noise and interference. The fading of channels caused by the multipath interference is closely related to the reflector or the user, i.e., the mobility of the user equipment. Transmitted signals and interference signals are received in a mixed state. Therefore, signals having become greatly distorted from the originally transmitted signals are received, which degrades the performance of the mobile communication system. As a result, the fading of channels may distort the amplitude and phase of a signal which is being received. Fading is a main cause of interrupting high-speed data communication in the wireless channel environment. Studies are under progress to solve the fading problem. In other words, it is required to reduce data losses caused by the characteristics of mobile communication channels, such as fading, and user interference to achieve high-speed data transmission in a mobile communication system. Diversity schemes are generally used as a method for preventing unstable communications due to fading. One diversity scheme, spatial diversity, uses multiple antennas.
The transmit antenna diversity scheme has emerged as an effective means for solving the fading problem. This scheme receives multiple signals, which have independently undergone fading in the wireless channel environment, and deals with a distortion caused by fading. The transmit antenna diversity scheme includes various diversity methods, such as frequency diversity, multipath diversity and spatial diversity. In other words, mobile communication systems should effectively solve the fading problem, which may seriously influence the communication performance, in order to achieve high-speed data transmission. The fading of channels reduces the amplitude of a signal, which is being received, by several dB to tens of dB. The diversity schemes mentioned above are utilized to solve the fading problem. For example, a Code Division Multiple Access (CDMA) method adopts a rake receiver which can obtain diversity performance using a delay spread of channels. The rake receiver is a receive diversity system which receives a multipath signal. However, the receive diversity scheme used in the rake receiver cannot obtain a desired diversity gain when the delay spread of channels is relatively small.
The time diversity scheme uses interleaving and coding to effectively compensate for burst errors generated in the wireless channel environment. The time diversity is generally used in Doppler spread channels. However, the time diversity scheme does not produce the diversity effect in low-speed Doppler channels. The spatial diversity scheme is generally used in channels with relatively small delay spread, for example, indoor channels and pedestrian channels which are low-speed Doppler channels. The spatial diversity scheme uses more than two antennas to obtain a diversity gain. When a signal transmitted through an antenna is attenuated by fading, signals transmitted through the other antennas are received to obtain a diversity gain. The spatial diversity scheme is divided into a receive antenna diversity scheme using a plurality of receive antennas and a transmit antenna diversity scheme using a plurality of transmit antennas. However, it is difficult to apply the receive antenna diversity scheme in view of the hardware minimization of the user equipment and the manufacture cost. Accordingly, it is generally recommended that the transmit antenna diversity scheme be used in the base station. The frequency diversity scheme obtains a diversity gain from signals which were transmitted in different frequencies and took different multiple paths. Since multipath signals have different fading information, the multipath diversity scheme obtains a diversity gain by separating the multipath signals. Further, the multipath diversity scheme allows coherent transmission in harmonization with the spatial channel characteristics and increases the SNR in proportion to the number of antennas.
The transmit antenna diversity scheme is divided into two schemes, i.e., closed-loop transmit antenna diversity, which uses downlink channel information fed back from the UE, and open-loop transmit antenna diversity, which does not use feedback information. The closed-loop antenna diversity scheme measures the channel phase and power of the UE to detect the optimum weight applicable to the channel of the UE. Therefore, the base station should transmit different pilot signals to the multiple antennas to measure the channel phase and power. The UE receives the pilot signals transmitted from the base station, measures the channel phase and power, and detects the optimum weight based on the measured channel phase and power.
Another method for enhancing the transmission capacity in a packet service communication system is antenna beam forming which uses a plurality of antennas, each having its own directivity to transmit signals. Beam forming also uses a nulling scheme to prevent a signal transmitted through one antenna from acting as an interference with signals transmitted through the other antennas. However, the nulling scheme for enhancing the throughput, which is significant in the transmission of data, such as packet data, is applicable only in antenna beam forming which limits the distances between antennas. The nulling scheme cannot be used in the transmit antenna diversity scheme which arrays antennas to be spaced from each another at a relatively long distance without limiting the distances between the antennas. In antenna beam forming, antennas are spaced from each another at a relatively short distance of λ/2. In the transmit antenna diversity scheme, antennas are arrayed at a much longer distance of 10λ. Since antennas far from each another lack correlation, the nulling scheme cannot be used in the transmit antenna diversity scheme.
Hereinafter, the beam forming scheme will be explained in further detail.
The beam forming scheme utilizes the nulling scheme based on the correlation between antennas spaced from each another at a relatively short distance of λ/2. As shown in Equation 1, an antenna weight is set to w1Hh2=0, w2Hh1=0 so that a receiving signal r1 of a first UE cannot receive data d2 of a second UE and a receiving signal r2 of the second UE cannot receive data d1 of the first UE.r1=(w1Hd1+w2Hd2)h1+n1=(w1Hd1+o)h1+n1r2=(w1Hd1+w2Hd2)h2+n2=(0+w2Hd2)h2+n2  Equation 1
If the channel condition is set to always generate a weight satisfying the requirements of Equation 1, the system will be able to completely null any influence of a channel on the other UE and double its capacity. In theory, nulling is possible whenever the number of UE to be subject to nulling in the beam forming environment, including a desired UE, is one less than the number of antennas. However, such a theory will be true if there is a change in phase only, while keeping the spatial correlation across antennas. Therefore, it is very difficult to implement the nulling scheme of general beam forming techniques in the wireless channel environment of mobile communications.
In the multiple antenna diversity scheme used to overcome channel fading, it is difficult to apply the nulling scheme because antennas lack correlation due to the long distance 10λ therebetween. For this reason, the base station generally uses a multiuser transmission scheme based on the orthogonality of spreading codes when simultaneously transmitting data to multiple users. Even in the multiuser transmission scheme, however, self-interference (SI) between multiple codes and multiple access interference (MAI) may occur when channels are subject to multipath fading. This may degrade the system performance. Therefore, it is necessary to apply nulling even in the multiuser transmission scheme.
As explained above, although it is important to use the nulling scheme for high-speed packet data services, the nulling scheme is applicable only in the beam forming technique which defines the distances between antennas. In general, when the antennas are spaced at a great distance, the correlation between signals of the antennas is reduced to rapidly increase the channel variation cycle, i.e., channel Doppler. Since a channel varies even within one frame of one user equipment in the existing voice communication environment, it is impossible to constantly null channels. Particularly, in a CDMA mobile communication system, antennas are provided in a number greater than the number of simultaneous users. It is almost impossible to apply the nulling scheme because of the excess of the degree of freedom for multiple antennas (i.e., number of antennas minus (−) 1). In other words, it is difficult to apply the nulling scheme in a CDMA mobile communication system where the preset frame is much longer than the coherent time, which is a channel variation section, and where the number of users who simultaneously access is greater than the number of antennas.
Hereinafter, transmit antenna array (“TxAA”), which is a scheme included in the closed-loop transmit antenna diversity scheme, will be explained in detail.
The TxAA scheme has two operation modes, a first TxAA mode (“TxAA Mode 1”) and a second TxAA mode (“TxAA Mode 2”). In TxAA Mode 1, the UE calculates weights W1 and W2, which will be used in UTRAN to maximize the receive power of a signal received by the UE, using a pilot signal transmitted from the base station. The calculated weights W1 and W2 are transmitted to the base station through a feedback information (FBI) field of a particular channel, for example, a dedicated physical control channel (DPCCH). Four weights 00, 01, 10 and 11 can be used in the UTRAN which operates in TxAA Mode 1. While TxAA Mode 1 adjusts the phase only, TxAA Mode 2 adjusts both the phase and the amplitude, i.e., every power information. There are 16 possible weights which can be used in the UTRAN. Each of the 16 weights has a value distinguishing a phase from an amplitude.
The weight w is a value relevant to a transmission channel. For example, w=h* (wherein w and h are vectors). Also, h refers to a transmit antenna array channel. Generally, in mobile communication systems using FDD (Frequency Division Duplex), transmission channels and receiving channels having different characteristics. In order to inform the base station of the transmission channel (h), the UE should feedback the transmit channel information to the base station. To this end, TxAA Mode 1 or TxAA Mode 2 are set to enable the UE to calculate the weight which will be obtained from the channel information (h) and feedback the weight information to the base station. TxAA Mode 1 quantizes and feedbacks only the phase component θ2-θ1 in the weight information (w=[|w1|exp(jθ1), |w2|exp(jθ2)]) (wherein w1 and w2 are scalar components). Thus, the phase precision becomes π/2, while the maximum quantization error becomes π/4. In addition, a refine mode which updates only one of two bits at every moment is used to improve the feedback efficiency. For example, combinations of bits can be {b(2k), b(2k−1)} and {b(2k), b(2k+1)} (wherein b represents a bit fed back per slot at every moment). TxAA Mode 2 feedbacks both the phase and the amplitude, which are components of the weight information. The phase is fed back in 3 bits, whereas the amplitude is fed back in 1 bit. Accordingly, the phase precision becomes π/4, while the maximum quantization error becomes π/8. Also, a progressive refine mode which updates only one of four bits at every moment is used to improve the feedback efficiency. In the refine mode, each bit is a value of orthogonal basis. However, the progressive refine mode does not have such a definition.
Communication systems supporting HSDPA transmit packet data in a particular unit, for example, in frames, only to a UE having the best channel condition at that time of transmission. In other words, HSDPA communication systems use the multiuser diversity scheme. The systems receive channel quality information from multiple UE which have requested HSDPA services, and determine the channel conditions of the multiple UE based on the received channel quality information. The systems select a UE having the best channel condition and transmit packet data to the selected UE only. Even when the system transmission capacity resource is large enough, the HSDPA communication systems transmit packet data to the selected UE only, thereby reducing the transmission efficiency. Also, as explained above, it is difficult to apply the nulling scheme in the HSDPA communication systems.